Objectives (1) To determine the relationship between male social status and reproductive success in captive social groups of Macaca arctoides; (2) to understand the mechanism(s) of reproductive suppression in males and its consequences for patterns of genetic transmission in stumptailed macaques; and (3) to understand the effect of environment and population density on male relationships in macaques. ABSTRACT:The foundation for future work on the relationship between male social status and reproductive success in stumptailed macaques was established by determining paternity of 27 infants born over an 8-year period in a large multi-male social group. The principal result was that the most dominant male of the group (alpha male) has a virtual monopoly on the production of offspring, with few exceptions (1 case in 27, attributed to inbreeding avoidance). In a second phase of the study, we attempted to evaluate the role of behavioural dominance, involving aggression and intimidation by the alpha male, in the effective reproductive suppression of subordinate males. A new "territory" was provided to the group, identical to their original enclosure and connected to it, with access to the new area restricted for the alpha male. Initial increase in sexual behavior by younger subordinate males (as well as females), out of view from the alpha, suggested that the inhibition of sexual activity in subordinate males may be principally behavioural in origin. Preliminary evidence of increased diversity in paternity of infants conceived following the environmental modification suggested that opportunity to avoid the alpha male may also disinhibit potential physiological suppression of reproduction in subordinate males. However, shortcomings of the technology involved in restricting movement of the alpha male require replication of the study with more sophisticated technology. To test the hypothesis that ability to escape aggression from the alpha male will lead to greater contribution of other males to the gene pool, we will regulate movement of animals through an automated door which reads implantable transponders and selectively filters passage of animals, barring the alpha male from the other half of the enclosure. Results bear directly on management strategies for the maintenance of genetic diversity in group-living mammals, and may have wide application in research and zoo environments. Also of interest are the behavioral strategies the alpha male may employ to control group members despite this restriction, and the behavioral choices that females will exhibit in this context, which simulates more natural conditions in which topography and space would prevent the alpha male's effective surveillance of other group members. Key Words reproductive competition, paternity, female choice, social behavior, colony management